1. Field of the Invention
The present invention relates to a duplex carburetor having primary and secondary barrels for supplying air-fuel mixtures under different operating conditions to an internal combustion engine, and an intake manifold for use with such a duplex carburetor.
2. Prior Art
Known duplex or two-barrel carburetors for internal combustion engines include a primary barrel for supplying an air-fuel mixture under a wide range of engine loads and a secondary barrel for supplying an air-fuel mixture under relatively high loads, the primary barrel including a primary venturi smaller in cross-sectional area than a secondary venturi in the secondary barrel. With certain ratios between the cross sections of the primary and secondary venturis, the speed at which an air-fuel mixture flows through the primary barrel becomes reduced as the pressure in the intake manifold is lowered while the secondary barrel is in operation, with the results that the fuel is less atomized than it should be in the primary barrel, and fuel supply at the secondary barrel is retarded, thus failing to provide smooth engine operation.
The primary and secondary barrels have circular cross sections and are spaced relatively widely apart from each other, the secondary barrel being located closer than the primary barrel to the combustion chambers of the engine. The primary barrel is connected to a primary common passage of a duplex intake manifold, which is detoured due to a secondary common passage of the intake manifold communicating with the secondary barrel. Thus, the primary passage is relatively long, presents higher resistance to the flow of an air-fuel mixture, fails to respond promptly to fuel needs, has a larger surface area which is conducive to an increased amount of fuel flow along the wall of the passage, and hence is unable to provide uniform fuel distribution among engine cylinders. As a consequence, combustion in the engine cylinders is rendered unstable for most operating conditions in which the engine usually works, resulting in a lower thermal efficiency of the engine and an increased amount of pollutants in the exhaust gas.